1. Field of the Invention
The present invention relates to a semiconductor storage device and, more particularly, to a semiconductor storage device whose storage capacity can be increased by being connected to a second semiconductor storage device.
2. Description of Related Art
In recent years, the development of semiconductor storage devices and, particularly, semiconductor storage devices that perform storage in a flash memory, which is a nonvolatile storage medium, has been carried out, and these semiconductor storage devices have come into widespread use as auxiliary storage devices of host equipment, such as personal computers, cell phones and digital cameras. With the capacity of data handled by host equipment having become large, large capacity designs and high density designs of flash memories also are going ahead. Among others, recent years have seen particularly frequently used NAND type flash memories.
A NAND type flash memory uses electric charges injected from a channel into a charge accumulation layer via an insulating film as information of digital bits, measures changes in the conductance of a field-effect transistor that depend on the charge amount, and reads out information. Unlike a DRAM, a NAND type flash memory is capable of readouts multiple times without data corruption.
Small semiconductor storage devices with excellent portability in which the NAND type flash memory is used can be broadly divided into a USB memory unit (hereinafter referred to as a “USB memory”) that is frequently used in personal computers (hereinafter also referred to as a “PCs”) and the like and a memory card that is frequently used in cell phones and digital cameras.
Most data retained in a USB memory and the like has hitherto been still images or music data with relatively small capacity and the like. For this reason, users were able to retain desired data without problems by using a semiconductor storage device having a capacity of 1 GB, for example. In recent years, however, users have come to be able to receive digital TV broadcasting by use of PCs and the like. When a 1-hour digital TV program is recorded with image quality equivalent to the image quality of DVD-Video, the data obtains a capacity on the order of 2 GB. Moreover, there is an increasing number of cases where users retain data in a small semiconductor storage device with excellent portability and carry the semiconductor storage device in order to watch digital TV programs in desired places. However, in the past in cases where users wanted to retain moving image data of high image quality and on other occasions, the retention was sometimes impossible because the capacity of the semiconductor storage device that permits retention was insufficient.
On the other hand, in NAND type flash memories, the storage capacity of one chip has been increasing at a double pace in one to several years due to the miniaturization of the wiring line width and the multiple-value storage method. For this reason, when new-type memory cards and the like whose storage capacity is twice to four times the storage capacity of old-type memory cards and the like have made their appearance, the old-type memory cards have been brought out of service before the expected product life thereof is reached.
Japanese Registered Utility Model No. 3100895 discloses a memory card reader/writer device in which a USB memory and various kinds of memory cards are connected to host equipment via one USB interface section.
In this memory card reader/writer device, however, though it is possible to connect a plurality of memory cards to one device, each of the connected plurality of memory cards is handled as a separate memory and hence it is impossible to retain one piece of data having such a large capacity as would exceed the capacity of individual memories. Furthermore, this memory card reader/writer device requires a large case and is very inconvenient in terms of portability. That is, this memory card reader/writer device is literally a mere connection interface device for use in a case where host equipment has no specific interface, such as a memory card, and reads data from the memory card and writes data on the memory card.